Figure 21. BGP Local PreferenceMulti-Exit Discriminators (MEDs)If two ASs connect in more than one place, a multi-exit discriminator (MED) can be used to assign a preference to a preferred path.MED is one of the criteria used to determine the best path, so keep in mind that other criteria may impact selection, as shown in theillustration in Best Path Selection Criteria.One AS assigns the MED a value and the other AS uses that value to decide the preferred path. For this example, assume the MED is theonly attribute applied. In the following illustration, AS100 and AS200 connect in two places. Each connection is a BGP session. AS200 setsthe MED for its T1 exit point to 100 and the MED for its OC3 exit point to 50. This sets up a path preference through the OC3 link. TheMEDs are advertised to AS100 routers so they know which is the preferred path.MEDs are non-transitive attributes. If AS100 sends an MED to AS200, AS200 does not pass it on to AS300 or AS400. The MED is a locallyrelevant attribute to the two participating ASs (AS100 and AS200).NOTE: The MEDs are advertised across both links, so if a link goes down, AS 1 still has connectivity to AS300 and AS400.Border Gateway Protocol IPv4 (BGPv4) 183
